According to conventional thinking, simple animals, including sponges, jellyfish and corals, evolved step-by-step in a linear fashion into those with more complex bodies, such as mammals.
按照传统的见解，包括海绵、水母和珊瑚在内的简单动物按部就班发生演变，以线性方式形成像哺乳动物一样的更加复杂的生物体。
youdao